There is well known a hybrid vehicle provided with: a differential mechanism having a first rotary element, a second rotary element serving as an input rotary member connected to an engine, and a third rotary element serving as an output rotary member; a first electric motor connected to the above-indicated first rotary element; and a second electric motor operatively connected to a power transmitting path from the above-indicated third rotary element to vehicle drive wheels. There has been proposed a technique to provide this type of hybrid vehicle with a locking mechanism configured to fix an output shaft of the above-indicated engine to a housing or any other stationary member. Patent Document 1 discloses a drive force output device as an example of this technique. According to this technique, the output shaft of the above-indicated engine is locked while the engine is held at rest (in a non-operated state), so that both of the above-indicated first and second electric motors can be used as a vehicle drive power source, whereby an output of the hybrid vehicle can be increased during running of the hybrid vehicle in a motor drive mode.